Field of the Disclosure
The present subject matter relates to radio frequency identification (“RFID”) devices. More particularly, the present subject matter relates to systems and methods for better reading a large population of RFID tags.
Description of Related Art
It is known to employ RFID technology to tag and identify individual pieces of merchandise. Typically, a plurality of RFID tagged items will be placed into a carton or similar container for shipment from a manufacturing or packaging facility to a retail location. Depending on the demands of the retail location, a plurality of cartons or containers may be delivered, with two or more cartons or containers shipped together on a pallet or the like.
Before the packaged items are shipped out of the manufacturing or packaging facility and/or when the packaged items arrive at the retail location, it may be advantageous to check the contents of the carton or container to ensure that the proper number of items are in the container, as well as the proper assortment of items. According to one known approach, handheld RFID scanning devices are used to catalog the contents of a carton or container.
A common protocol for reading UHF RFID tags is EPC Class 1 Generation 2, also known as ISO 18000-6C. As part of the communication protocol between an RFID reader and RFID tags, a plurality of RFID tags in a large group or population are read or scanned with an anti-collision algorithm in effect. Ideally, an RFID tag that has been read in a round of inventorying should not respond again in the same round unless specifically instructed to do so. To achieve this functionality, an RFID chip of the RFID tag may include was may be referred to as a “session flag,” which has two states that may be referred to as “A” and “B.” A round of inventorying may be carried out in such a way as to read all RFID tags having a session flag in the “A” state, with the RFID tags that have been read moving from the “A” state to the “B” state and not responding to the signal from the RFID reader for a given time (e.g., one second or fifteen seconds or more) to prevent them from being read a second time in a given round.
One possible difficulty when using an RFID reader or scanning device to read all of the RFID tags in a carton or container or in some other grouping is a phenomenon referred to herein as “proximity coupling.” When RFID tags are closely spaced together in a large group (e.g., with a separation of 25 mm or less between adjacent RFID tags), there is a significant interaction between the various tags, especially for identically tuned tags. Exemplary effects of proximity coupling may include reduction in sensitivity at the desired operating frequency, reduction in backscattered signal, and changes in the direction of maximum sensitivity. While the above-described protocol of switching states of an internal flag is sufficient to prevent a single RFID tag from being read twice in the same round, an RFID tag not responding to a signal does not remove its impact on the performance of other RFID tags in its area (especially those tuned to the same basic frequency).